In the field of semiconductor substrates, such as SeOI and sSOI substrates, particularly for microelectronics, optoelectronics, etc:, it is well known to form an electrically insulating layer, such as an oxide or nitride layer, on at least one of the substrates in order to bond the two substrates brought into contact with each other during their fabrication or to protect the surface of the substrate against chemical contamination and scratching, in particular while they are being manipulated.
The oxide layer is formed either by direct oxidation, such as thermal or anodic oxidation, or by deposition of an oxide layer, the formation of a nitride layer being produced by deposition.
According to the first process for forming an oxide layer, said thermal oxidation is obtained in a furnace in which substrates, for example made of silicon, are submitted to a temperature of between 900 and 1200° C.
A gas is introduced into the furnace to oxidize the substrates, an oxide layer growing with time on said substrates. In the dry thermal oxidation process, the gas is oxygen and in the wet thermal oxidation process, the gas is water vapour.
The thermal oxidation process, and more particularly the dry thermal oxidation process, gives an oxide layer which has a very high quality, is particularly dense and has a high dielectric strength.
However, the drawback of process is the low rate of growth of the oxide layers.
The second type of process for forming an oxide layer, namely by deposition, includes several processes for depositing an oxide layer which are well known to those skilled in the art.
Most of these processes consist in particular of low-temperature CVD (Chemical Vapour Deposition), LPCVD (Light-Pressure Chemical Vapour Deposition) or PECVD (Plasma Enhanced Chemical Vapour Deposition) processes.
These oxides deposition are formed in a furnace in which for example silicon substrates are submitted to a temperature of between 300 and 800° C.
These processes allow the formation of thick oxide layers on the substrates in a relatively short time.
However, theses types of oxide layers are porous. The porosity of these oxide layers results in a low density of the layer, which impacts the quality of the transfer obtained by Smart Cut™ method, method described in the publication “Silicon-On-Insulator Technology: Materials to VLSI”, by Jean-Pierre Colinge, 2nd edition, published by Kluwer Academic Publishers, pages 50 and 51.
The transfer of a layer by the Smart Cut™ method may be of poor quality when the deposited oxide layer has not been densified by a heat treatment.
The densification of these oxide layers can be obtained by application of an annealing at a temperature between 600 and 1200° C., for between 10 minutes and 6 hours, as described in the international patent application WO 2006/029651. This heat treatment also allows certain species, such as for example carbon, incorporated into the oxide during deposition, to be degassed.
However, during the fabrication of multilayer heterostructures, application of this heat treatment to densify an oxide, layer deposited on the multilayer substrate is limited to low-temperature ranges to avoid the appearance of defects and the diffusion of the species between the various layers of the substrate.
Specifically, during the various annealing steps during the Smart Cut™ method, the degassing of the deposited oxide layer may result in the formation of defects.
It should be noted that the same drawbacks occur in the case of nitride insulating layers.